This application claims the priority of German patent document 10 2008 025 064.3-55, filed May 26, 2008, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to a process for improving continuity in a two-frequency navigation satellite system.
Global Navigation Satellite Systems (GNSS), sometimes referred to as Navigation Satellite Systems, are used for position indicating and navigation on the ground and in the air. GNSS Systems, such as the operating GPS (Global Positioning System), or the European Navigation Satellite System (hereinafter, also called Galileo System, or abbreviated Galileo), which is being constructed, have a satellite system (space segment) comprising multiple satellites, an earth-fixed receiving device system (ground segment), which is connected with a central computing station and includes several ground stations, as well as Galileo sensor stations. Utilization systems evaluate and utilize the satellite signals transmitted from the satellites by wireless communication, particularly for navigation. In the GPS, satellite signals are transmitted for civil purposes in the L1 frequency band (that is, with a 1575.42 MHz carrier frequency). As a result, propagation time errors caused by the effects of the ionosphere may directly impair the navigation precision.
In the Galileo System, which is currently under construction, and in the planned modernization of the GPS, it is contemplated that two frequency bands will be used for the transmission of civil satellite signals, in order to better compensate or even eliminate the influence of the ionosphere on the propagation time of the satellite signals. In such a two-frequency navigation satellite system, the same satellite signal is transmitted in two different frequency bands, for example, in the L1 and in the L2 band (1227,60 MHz).
A user system such as a mobile navigation device which receives the signal can compare the two signals and compensate propagation time differences. However, the failure of the transmission of satellite signals in one of the two frequency bands may negatively affect the continuity of the system because a receiving user system can no longer determine the influence of the ionosphere. This can be a disadvantage particularly for the safety-critical service (Safety of Life (SoL) Service) provided by the Galileo System, which requires a continuously high navigation precision.
It is therefore an object of the present invention to provide a process for improving continuity in a two-frequency navigation satellite system.
This and other objects and advantages are achieved by the process and apparatus according to the invention, in which the ionosphere is observed by two-frequency measurements and changes of the ionosphere are signaled. A user system can thereby be warned of particularly strong changes of the ionosphere and a resulting possible interference with the transmission of a satellite system and can react correspondingly, so that the continuity can be improved. In particular, the failure of the transmission of satellite signals on one frequency because of strong changes of the ionosphere no longer leads necessarily to a continuity event because a user system can react sufficiently quickly to a signaled change of the ionosphere, and still complete its operation.
Accordingly, one embodiment of the invention now relates to a process for improving the continuity in the case of a two-frequency navigation satellite system, which process includes the following steps:                Observing the ionosphere by measurements in the two or more frequency bands, and        transmitting an alert message which informs user systems of a change of the ionosphere when at least one measurement indicates a change that deviates from one or more predefined conditions. This technique makes it possible to alert a user system, which can adapt its position indication to take into account the change of the ionosphere.        
According to a feature of the invention, the observation of the ionosphere by measurements in the two or more frequency bands can take place from a ground segment of the navigation satellite system and/or from satellites of the navigation satellite system. Also, the transmission of the alert message can take place either by way of satellites of the navigation satellite system or from a ground segment of the navigation satellite system.
The measurements can include propagation time measurements of signals transmitted in the frequency bands, so that a change of the ionosphere can be determined relatively rapidly and reliably.
The observation of the ionosphere by measurements in two or more frequency bands may comprise the emission of at least one measuring signal. Instead of conventional signals of the navigation satellite system, which can also be utilized for the observation of the ionosphere, a targeted observation of the ionosphere can also be carried out using separate measuring signals.
According to the invention, one of the predefined conditions may be a minimum duration of a change of the ionosphere. Therefore only a measurement or observation of the ionosphere which indicates a longer (rather than transient or fast) change of the ionosphere can trigger the emission of an alert message because the continuity could be restricted by a longer change.
Another one of the predefined conditions may be an exceeding of a predefined maximum propagation time deviation of a measuring signal. As a result, brief propagation time deviations of a signal from an expected propagation time (which may be caused, for example, by fast ionosphere changes) cannot cause emission of an alert message which could lead to unnecessary reactions in the user systems.
The alert message according to the invention may have one or more of the following pieces of information: Region with respect to which there is a disturbance of the ionosphere; location and extent of an ionospheric disturbance in a layer at a certain altitude; a polygon which indicates the region of an ionospheric disturbance in a layer at a certain altitude; a mere indication of the presence of a disturbance; information on a delay or a reduction of a delay maximal in a time interval, for which continuity is to be ensured.
The invention also relates to a two-frequency navigation satellite system, which comprises a space segment having several satellites which emit satellite signals containing navigation messages for the reception and evaluation by user systems for position indication and navigation by way of two frequencies, and a ground segment having several observation and command stations which monitor the satellites, one or more observation and command stations and/or satellites for implementing the process according to the invention as described above in order to optimize the continuity in the two-frequency navigation satellite system.
Another embodiment of the invention relates to a process for processing an alert message, having the following steps:                Receiving an alert message which was transmitted by means of the process according to the invention as described above;        determining the information concerning a change of the ionosphere contained in the alert message; and        computing the influence of the change of the ionosphere on a position indication. This process can be implemented, for example, in the form of an algorithm, in a receiver for satellite signals.        
The process according to the invention can be carried out by a receiver for satellite signals of a two-frequency navigation satellite system. If the receiver first receives two measuring signals on two different frequencies and then loses one of the measuring signals, so that only one of the two measuring signals will still be available, it uses a last-determined ionospheric propagation time delay of the particular measuring signal which is no longer available for the position indication. In addition, in this embodiment, the process also includes a step of deciding whether the last-determined ionospheric propagation time delay of the measuring signal can be used to compensate the influence of the change of the ionosphere received by means of the alert message. This makes it possible for a receiver still receiving only one measuring signal on one frequency (because, for example, there is a disturbance of the second frequency provided for the transmission of satellite signals) to decide upon receipt of an alert message whether the ionospheric disturbance signaled by means of the alert message can still be compensated by means of the last-determined propagation time delay, or whether it is so large that compensation using the last-determined propagation time delay no longer makes sense in order to maintain the required continuity.
Furthermore, according to another embodiment of the invention, the process includes a further step of excluding those satellites from the position indication which, as a result of the computed influence of the change of the ionosphere on the position indication, have signal propagation times that are too long.
Finally, the invention also provides a receiver for signals of a navigation satellite system which contain alert messages, wherein the receiver is configured to perform the process for processing an alert message according to the invention and as described above. The process can, for example, be implemented in the operating software of a receiver for navigation messages, such as a mobile navigation device. As a result, the functionality of the receiver can be expanded in that, in the event of a change of the ionosphere signaled by means of an alert message, the receiver initiates measures to maintain the precision of the position indication.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.